[Sec 2 Maths: Pythagoras' Theorem]
The following question features an important problem-solving heuristic that is beneficial for many students.
You can assume an unknown length using algebra, and solve for unknowns using formulae you've learned before. This is a vital problem-soling heuristic that can be used for many, MANY questions in the future.
Make sure to practice with this as much as you can!